1. Field of the Invention
This invention relates generally to a communication system and, more particularly, to a communication system for broadcasting to mobile users.
2. Discussion of the Related Art
For communication systems in which information is broadcast to mobile users, a key challenge in such systems is overcoming signal degradation effects associated with mobile propagation. The mobile user is subject to a time-varying environment that includes attenuation from physical blockages (i.e., buildings, trees, terrain, etc.), as well as multipath and interference effects. One way of overcoming these effects is to increase the transmit power beyond what is required for clear-line-of-sight (CLOS) operation in a multipath-free and interference-free environment. However, due to the large amount of additional transmit power often required, this is not always feasible for practical communication systems. This is particularly relevant for satellite-based communication systems broadcasting to mobile users. Although, there is generally less blockage for satellite-based systems than terrestrial-based systems due to the higher elevation angles, it is still difficult in a satellite-based system to transmit enough power to overcome the signal attenuation effects, particularly in outer fringe areas of the coverage region.
Communication systems are also increasingly expected to be capacity efficient and heavy demands are currently required on many different types of digital communication channels. With many of these communication channels, a relatively large amount of bit errors may occur because of the noted physical blockage in a relatively short period of time within a sequence of transmitted bits. Errors occurring in this manner are generally referred to as burst errors, and thus, such communication channels, particularly mobile communications channels, are typically referred to as bursty or fading channels.
Consequently, communication systems operating in the mobile propagation environment have both time and spacial dependencies. Conventional communication systems may thus employ coding and interleaving, as well as clear-line-of-sight link margins to combat the time and spatially vary mobile propagation environment. Although such techniques may be somewhat effective in this environment, current implementations are not very efficient in terms of the way they utilize the available power, bandwidth and receiver resources, such as receiver memory.
Traditional coding methods for communication systems operating through a bursty or fading channel often employ some form of interleaving in order to make the communication system more reliable. As is known in the art, interleaving attempts to spread the effect of burst errors in time such that the bit errors are decorrelated and separated from one another. This repositioning of error bits tends to separate the error bits so that they can be processed in conjunction with an encoding and decoding communication system. A convolutional or block decoder is able to tolerate up to some fraction of its input bits degraded or erased, known as the decoder's erasure threshold, and still provide acceptable performance, measured by bit error probability. The purpose of the conventional interleaver is thus to reduce the probability that the decoder's erasure threshold is exceeded.
Conventional uniform interleavers have an input-to-output delay distribution that is uniform in probability over an interval from a minimum delay to a maximum delay or length of the interleaver. When optimizing the performance of a conventional uniform interleaver, the only major trade involving the interleaver is its delay or length. If the interleaver is much longer than the correlation time of the channel, which is generally defined as the separation time at which fading probabilities become uncorrelated, the probability of a burst error at the output of the interleaver is small. The maximum delay or length of the interleaver is thus generally related to the fade correlation time of the channel. Although a uniform interleaver is not necessarily optimized for a particular channel, it is generally effective when the maximum delay or length is chosen to be much larger than the fade correlation time of the channel.
The coding parameters are also generally chosen based on the assumption that the interleaver/deinterleaver has managed to completely decorrelate fading events over time. In reality however, the length of a uniform interleaver necessary to sufficiently decorrelate common mobile fading channels becomes unreasonably long. This puts excessive memory requirements on the receiver and has the additional disadvantage that it slows down the acquisition process. As the fade correlation time becomes even longer, it is generally not feasible to make a uniform interleaver of sufficient delay or length to provide adequate decorrelation of the faded information. With longer delays or interleaver length, a delay in data acquisition becomes even longer. Such an information delay degrades the quality of real time signals, especially in two-way mobile voice communications. Consequently, long uniform interleavers are not well-suited to mobile communication systems that require fast acquisition. Moreover, simple retransmission of data as is used in some existing systems is an inefficient means for dealing with burst errors in the channel.
As far as the link margin is concerned, some conventional systems use link margin as their primary means of fade mitigation. This may be conceptually effective, but is an extremely inefficient use of transmit power resources and in fact many systems cannot afford to provide this much excess transmit power. Even if link margin is coupled with coding/interleaving, often the system is designed for a single minimum link margin over the coverage region. It is typically based on the worst case propagation statistics over the coverage region. The actual link margin will just be whatever results from standard transmit and receiver antenna gain patterns used to cover the region with the minimum link margin. There is no effort to match the link margin to the geographically varying propagation environment, so often this results in excessive link margins in the areas that don't need it, and having just enough in areas that need it the most. In the end, such use is an inefficient means of allocating available power.
What is needed then is a communication system for broadcasting to mobile users having a structure of coding/interleaving and link margins designed specifically to match the time and spatial dependencies of the propagation environment such that it will not suffer from the above mentioned disadvantages. This will, in turn, result in superior performance in the mobile environment with efficient use of power, bandwidth and receiver resources; minimize expected correlation between deinlterleaver output bits; increase data acquisition times; increase quality of real time signals; decrease the probability that the average erasure fraction exceeds the decoder's threshold; remove the restriction of a uniform interleaver delay; matches the length and shape of the interleaver, as well as the rate of the encoder to channel correlation statistics; provides an overall communication system specifically designed for the signal propagation characteristics of mobile users; minimizes outages in a power efficient manner; provides reliable communication to mobile users; and intentionally weights system link margins across the coverage region where the mobile users operate to counteract regional variations in the propagation environment. It is, therefore, an object of the present invention to provide such a communication system for broadcasting to mobile users.